Launching to specific LANs

[FTLparachute]

Currently planning out a mission that will require me to launch a rocket into a Kerbin orbit with a specific LAN

I have no clue how I am supposed to get the LAN right and would like to get good at this.

Any recommendations?

EDIT - Just realised that since the LAN is taken from a fixed reference point, i should probably also ask where this reference point is and how to find it

Edited Thursday at 10:02 PM by FTLparachute

[Fizzlebop Smith]

I do not recall where I found these instructions. The have been saved in my notes for 18 months now and thought they would be helpful.

Forgive me but I asked an automation tool to clean it up for a form post.

 

1. Understand Key Orbital Terms:

• Longitude of Ascending Node (LAN):
  This is the angle, measured from a reference direction (in KSP, typically from the prime meridian at longitude = 0°), indicating where your orbit crosses from south to north over the equator.
• Inclination:
  The angle between your orbit plane and the planet’s equator.

2. Planning Your Launch Window:

The trick to hitting a specific LAN is timing. You need to:

• Launch when your launch site passes directly underneath the intended orbit plane.
• Use KER (Kerbal Engineer Redux) or MechJeb (optional but highly recommended) to see real-time orbital data and planet rotation relative to your target orbit.

Optimal Timing:

• Warp time until your launch site aligns closely underneath the orbit line on your map.

(This is the most critical part. Launch site alignment with end goal. I ALWAYS have to perform minor orbital maneuvers to get the LAN perfect)

• If you're launching from Kerbal Space Center (KSC), wait until KSC rotates under your target orbit’s path on the map screen (watch from the Tracking Station or Map view).

3. Setting Inclination at Launch:

• Before liftoff, note if the orbit line moves slightly north or south relative to your launch site.
  • If the orbit line moves northeast, launch into a heading slightly northward of the standard eastward (typically around 90°) direction (e.g., heading of 50°-70°).
  • If the orbit line moves southeast, aim slightly south of eastward (e.g., heading of 100°-120°).

A rough heading-to-inclination guideline:

• 0° heading: Directly North
• 90° heading: Directly East (equatorial orbit)
• 180° heading: Directly South
• 270° heading: Directly West (retrograde orbit, rarely useful)

Adjust heading slightly during ascent to match target inclination precisely.

4. During Launch (Gravity Turn and Adjustments):

• Start your gravity turn normally (around 50-100 m/s vertical speed), but angle immediately towards the determined heading (north or south of the 90° standard East).
• Watch the Ascending Node (AN) and Inclination indicators provided by KER or MechJeb during ascent to fine-tune your trajectory.
• Small adjustments early on save huge delta-v later.

5. Circularize as Close to AP as Possible

6. Achieving Exact LAN:

For precise LAN targeting, remember the planet rotation:

• Kerbin rotates roughly 1° every 60 seconds (1°/minute, 360° rotation every ~6 hours).

 

• Adjust your launch time if your first attempt misses the LAN by a known angle.
  Example: If your orbit ends 10° off your desired LAN, adjust launch by ~10 minutes forward or backward.

I usually eye ball the intersect for a normal / radial translation to the proper site but you can relaunch until you get better at it.

---

Example (Practical Summary):

• Check contract: Inclination 30°, LAN = 120°.
• Open map view, find when KSC aligns under orbit line (around LAN=120°).
• Launch at this exact time, aim for approximately 60° heading (30° inclination).
• Fine-tune on ascent.
• Minimal circularization burn to stabilize orbit.

Hope this Helps

[FTLparachute]

6 minutes ago, Fizzlebop Smith said:

I do not recall where I found these instructions. The have been saved in my notes for 18 months now and thought they would be helpful.

Forgive me but I asked an automation tool to clean it up for a form post.

 

1. Understand Key Orbital Terms:

• Longitude of Ascending Node (LAN):
  This is the angle, measured from a reference direction (in KSP, typically from the prime meridian at longitude = 0°), indicating where your orbit crosses from south to north over the equator.
• Inclination:
  The angle between your orbit plane and the planet’s equator.

2. Planning Your Launch Window:

The trick to hitting a specific LAN is timing. You need to:

• Launch when your launch site passes directly underneath the intended orbit plane.
• Use KER (Kerbal Engineer Redux) or MechJeb (optional but highly recommended) to see real-time orbital data and planet rotation relative to your target orbit.

Optimal Timing:

• Warp time until your launch site aligns closely underneath the orbit line on your map.

(This is the most critical part. Launch site alignment with end goal. I ALWAYS have to perform minor orbital maneuvers to get the LAN perfect)

• If you're launching from Kerbal Space Center (KSC), wait until KSC rotates under your target orbit’s path on the map screen (watch from the Tracking Station or Map view).

3. Setting Inclination at Launch:

• Before liftoff, note if the orbit line moves slightly north or south relative to your launch site.
  • If the orbit line moves northeast, launch into a heading slightly northward of the standard eastward (typically around 90°) direction (e.g., heading of 50°-70°).
  • If the orbit line moves southeast, aim slightly south of eastward (e.g., heading of 100°-120°).

A rough heading-to-inclination guideline:

• 0° heading: Directly North
• 90° heading: Directly East (equatorial orbit)
• 180° heading: Directly South
• 270° heading: Directly West (retrograde orbit, rarely useful)

Adjust heading slightly during ascent to match target inclination precisely.

4. During Launch (Gravity Turn and Adjustments):

• Start your gravity turn normally (around 50-100 m/s vertical speed), but angle immediately towards the determined heading (north or south of the 90° standard East).
• Watch the Ascending Node (AN) and Inclination indicators provided by KER or MechJeb during ascent to fine-tune your trajectory.
• Small adjustments early on save huge delta-v later.

5. Circularize as Close to AP as Possible

6. Achieving Exact LAN:

For precise LAN targeting, remember the planet rotation:

• Kerbin rotates roughly 1° every 60 seconds (1°/minute, 360° rotation every ~6 hours).

 

• Adjust your launch time if your first attempt misses the LAN by a known angle.
  Example: If your orbit ends 10° off your desired LAN, adjust launch by ~10 minutes forward or backward.

I usually eye ball the intersect for a normal / radial translation to the proper site but you can relaunch until you get better at it.

---

Example (Practical Summary):

• Check contract: Inclination 30°, LAN = 120°.
• Open map view, find when KSC aligns under orbit line (around LAN=120°).
• Launch at this exact time, aim for approximately 60° heading (30° inclination).
• Fine-tune on ascent.
• Minimal circularization burn to stabilize orbit.

Hope this Helps

Thanks. Didn't think about changing timing to fine tune the orbit - that's going to save me a lot of troubles

Did my own calculations to find when I would need to launch, as i managed to figure out that 0,0 was pointing at the reference direction

Spoiler

LAN+74 gives the angle of KSC relative to the reference direction

Angle over 360 gives fraction of day needed for KSC to intersect ascending node

Fraction multiplied by 21600 gives the fraction in seconds from 0h, 0m, 0s

i.e: LAN of 137 would need a launch at 3h, 31m and roughly 34 seconds

[jimmymcgoochie]

MechJeb > ascent guidance > launch to LAN.

[Alex-Frst]

You can try my calculator: